Interferometers capable of measuring distance are well known in the prior art. Although the present invention is applicable to interferometers generally, one type of interferometer representative of the current state of the art is described in Bagley et al., U.S. Pat. No. 3,458,259, issued July 26, 1969, and this type of interferometer is shown in conjunction with the specific embodiment of the invention described and claimed hereinafter.
As noted above, distance measuring interferometers are subject to environmental effects, particularly to changes in air pressure, temperature and humidity. Such factors tend to alter the wavelength of the beam of light used by the interferometer to measure distance.
In the past, it has been possible to correct the environmental errors by using individual sensors for the barometric pressure, temperature and humidity and using the measurements for the correction of the variable length interferometric measuring path. This has been only partly satisfactory due to the sensor unreliablilty, their errors and errors of their application.
The present invention solves this problem by directly measuring these effects upon an optical path of fixed length and applying the measurement to correct the corresponding errors of the variable length path.
It is a further purpose of the present invention to automatically correct the distance measuring interferometer by producing a signal indicative of the number of interferometer light wavelengths in a path of predetermined fixed length, and applying that signal to correct the indicated number of interferometer wavelengths in a measuring path of variable length.